This invention generally relates to a torque converter associated with a vehicle transmission. More particularly, the present invention pertains to a control device of a lock-up mechanism in a torque converter of a vehicle for controlling the slip value defined as the difference between the rotation speed of a pump impeller connected to the engine and the rotation speed of a turbine impeller connected to a wheel side element.
Generally speaking, a lock-up mechanism used in the torque converter of a vehicle controls the slip value between the rotation speed of the pump impeller and the rotation speed of the turbine impeller by controlling the force applied to a piston while controlling the hydraulic pressure into the torque converter.
A Japanese patent application published as Toku-Kai-Hei 8(1996)-28681 discloses a control device of a lock-up mechanism which controls the slip value of the pump impeller and the turbine impeller. This control device controls the actual slip value so that it coincides with a transient target slip value, so that the actual slip value approaches a target slip value gently. More specifically, the control device calculates the target slip value (fixed value) based on a driving condition of the vehicle and sets an initial value of the transient target value based on the actual slip value immediately before operation of this device is initiated. The transient target slip value is reduced toward the target slip value by a predetermined decrement which decreases gradually as the transient target slip value approached the target slip value as shown in FIG. 14 of the published application. Accordingly, the actual slip value approaches the target slip value gently.
However, situations arise in which the actual slip value overshoots the target slip value because the transient target slip value is uniformly reduced by the predetermined decrement independently of the dispersion of the target slip value calculated based on varied driving condition of the vehicle. This occurrence of the overshoot deteriorates the drivability of the vehicle.
A need thus exists for a vehicle lock-up mechanism control device which is not as susceptible to the same disadvantages and drawbacks as those mentioned above.
To achieve the aforementioned object the following technical means is provided with a control device of a lock-up mechanism includes the lock-up mechanism arranged in parallel with a hydraulic type torque transmitting mechanism transmitting a rotatory power of a pump impeller connected with an output shaft of an engine to a turbine impeller connected with a wheel side element, the lock-up mechanism controlling a slip value between a rotational speed of the pump impeller and a rotational speed of the turbine impeller in response to a hydraulic pressure supplied thereto, a target slip value calculating means for calculating a target slip value based on driving condition of a vehicle, an actual slip value detecting means for detecting an actual slip value from the difference between the rotational speed of the pump impeller and the rotational speed of the turbine impeller, an intermediate slip value setting means for setting an intermediate slip value between the actual slip value and the target slip value, a hydraulic pressure controlling means for controlling the hydraulic pressure supplied to the lock-up mechanism in order for the actual slip value to coincide with the intermediate slip value, and an intermediate slip value renewal means for reducing the intermediate slip value to a renewed value closer to the target slip value than the intermediate slip value when the actual slip value reaches the intermediate slip value.
According to this invention, the intermediate slip value is set between the actual slip value and the target slip value. Namely, the intermediate slip value is certainly set at a slip value larger than the target slip value. It is thus possible to control the actual slip value so as not to overshoot the target slip value.
Another aspect of the invention involves a method for controlling a lock-up mechanism arranged in parallel with a hydraulic type torque transmitting mechanism that transmits rotatory power of a pump impeller connected with an output shaft of an engine to a turbine impeller connected with a wheel side element, with the lock-up mechanism controlling a slip value between a rotational speed of the pump impeller and a rotational speed of the turbine impeller in response to hydraulic pressure supplied to the lock-up mechanism. The method includes calculating a target slip value based on driving conditions of a vehicle, detecting an actual slip value between the rotational speed of the pump impeller and the rotational speed of the turbine impeller based on the difference between the rotational speed of the pump impeller and the rotational speed of the turbine impeller, setting an intermediate slip value between the actual slip value and the target slip value, controlling hydraulic pressure supplied to the lock-up mechanism to cause the actual slip value to approach the intermediate slip value, and reducing the intermediate slip value to a renewed value closer to the target slip value than the intermediate slip value when the actual slip value reaches the intermediate slip value.